1. Field of the Invention
This invention relates to the heat treatment of glass sheets while they are supported in an upright disposition. For example the invention relates to the heating of glass sheets for subsequent toughening, bending, or bending and toughening, or prior to annealing or coating of the glass sheets.
2. Background of the Invention
It is usual to suspend glass sheets vertically from tongs and convey the suspended glass through a heating furnace prior to toughening or bending. In another method of heat treating glass sheets which are in an upright disposition cushions of hot gas have been formed on the surfaces of the sheet to support the upright sheet while it is advanced on a conveyor by engagement with the lower edge of the sheet.
Glass sheets have been heated and quenched while conveyed horizontally on a horizontal roller hearth through a furnace in which the glass is heated to a pre-quenching temperature, and then between blowing boxes which direct cold gas, usually air, at the upper and lower surfaces of the glass to toughen the glass.
The pre-quenching temperature to which the glass is heated may be as high as 680.degree.C to 700.degree.C, that is to a temperature just below the softening point of the glass. When at a high temperature just as the glass leaves the furnace it is susceptible to damage by marring of the glass surface by its contact with the rollers, and by sagging of the glass between the rollers due to gravitational forces.
In order to avoid the disadvantages of supporting the glass on horizontal rollers as it is heated and quenched, a gas support system has been developed in which the glass is supported on gas flows while its temperature is such that deformation of the glass is possible. In this system glass sheets are heated and quenched while horizontally disposed on a substantially uniform gaseous support provided by continuously maintaining upward gas flows beneath the sheet at a rate to provide a gaseous cushion beneath the sheet. The glass is moved along a horizontal treatment path while so supported firstly on hot gas which heats the glass to a pre-quenching temperature and then on cold gas which quenches the lower surface of glass at the same time as the upper surface of the glass is quenched by complementary downward flows of cold gas.
In such a system the only mechanical contact with the glass is an edge contact which drives the glass forwardly along a horizontal treatment path. Contact with the surfaces of the sheet is minimal and the support is more uniform than a roller support so that there is less possibility of introducing distortion into the sheet.
Such gaseous support systems are however costly and difficult to regulate since minimal controlling contact with the hot sheets is desirable.
It has now been found that glass sheets can be heated while supported mechanically in an upright position by leaning against a transient mechanical support, for example spaced rollers which are inclined at a small angle to the vertical as long as the support against which the glass leans is advancing at the same forward speed as the glass sheet itself so that no relative slipping takes place between the mechanical support and the glass. Contact of the glass surfaces with the roller surfaces may only be over small areas of the sheet and as the sheet is upright, gravitational loading between the glass surface and the surfaces of the rollers is minimal so that there is a greatly reduced risk of marking of the sheet as compared with that customary with a horizontal roller support. Further since the gravitational load on any part of the sheet, particularly when it is approaching its softening point, is kept to a minimum, there is less risk of sagging of the sheet and any sagging of the sheet between adjacent rollers is picked up by the next roller.
It is a main object of the present invention to provide a method and apparatus for heat treating glass while it is supported in an upright disposition by leaning at a small angle to the vertical against mechanical supporting means.